6.6% lower difficulty with 1/3 hashing power.

Let's say you have 1/3 mining power, and you consistently divide the hashing power of the network by releasing blocks with timestamps that will get ignored by 1/5 of the network, but not ignored by 4/5 of the network, of which you are a part.

The 1/5 has a 1/16 chance of advancing the chain, which will cost you one block.
The 4/5 has a 15/16 chance of advancing the chain.

1/3 of the time you are causing 1/5 of the network to mine on what will become a stale chain with 15/16 probability. This means that the main chain is going to have ~1/15 less hashing power, resulting in a difficulty that's 6.6% lower. This 6.6% lower difficulty weakens the chain, but empowers you because you're only losing 1/16th of the blocks you mine while performing this attack, but gaining 1/15th in blocks as a result of lowering the difficulty.

Using this attack means you can lower the difficulty by 6.6% while actually increasing your profits, to the detriment of the 1/5 miners that continue to ignore your high timestamp blocks.